This invention relates to release coatings containing perfluorocarbon release agents and to processes for using such coatings to prevent deposition of undesirable materials on certain substrates.
A wide variety of chemical processes are commonly carried out in large, stirred vessels which are frequently provided with auxiliary equipment such as baffles and heat transfer coils which enable heat to be supplied or extracted from the contents of the vessels. In many cases, however, such processes eventually produce undesirable deposits on the surfaces of the equipment with which the reaction mixtures come into contact. Such deposits interfere with the efficient transfer of heat to and from the interior of the vessels. Sometimes, these deposits have a tendency to deteriorate and to partially fragment, resulting in contamination of the reaction mixture and the products produced therefrom. This problem is particularly prevalent in polymerization since the deposits or "build-up" of solid polymer on reactor surfaces, not only interferes with heat transfer, but decreases productivity and adversely affects polymer quality.
This problem of polymer "build-up" is particularly severe in the commercial production of olefin polymers such as polyethylene, polypropylene and copolymers of ethylene with other olefins such as propylene, 1-butene, 1-hexene and 1-octene as well as in the polymerization of many other ethylenically unsaturated monomers such as vinyl chloride, vinylidene chloride and similar monomers having a terminal ethylenic group. As an example of particular interest, linear ethylene polymers are produced commercially by contacting a monomer stream containing ethylene with a metal catalyst such as a Ziegler catalyst dispersed in liquid hydrocarbon. When such polymerizations are carried out under slurry conditions, the polymer is produced as a solid particulate which is dispersed or slurried in the liquid hydrocarbon by agitation. Unfortunately, such slurries or dispersions are inherently unstable; thus, during polymerization, ethylene polymer often builds up upon interior surfaces of the baffles and the agitator. This undesirable polymer deposit deleteriously affects heat transfer, polymer quality and productivity.
Accordingly, in the commercial practice of such polymerization processes, it is necessary to periodically stop the reaction and have production personnel enter the reactor and remove polymer deposits from reactor walls, baffles, agitator, etc. This cleaning operation is not only costly, both in labor and down-time of the reactor, but can be dangerous to the persons cleaning the reactor. While various methods, such as solvent cleaning, have been proposed heretobefore to reduce the amount of polymer build-up on polymerization reactor walls, none have been entirely satisfactory.
More recently, as disclosed in U.S. Pat. Nos. 3,825,434; 3,849,179 and 4,007,320, those skilled in the polymer arts have attempted to reduce polymer build-up on internal surfaces of the reactor by coating such internal surfaces with a hydrophilic polymer that is cross-linked to render it insoluble in the reaction medium. Unfortunately, these coatings often (1) do not adhere well to the metallic internal surfaces of the reactor, (2) deactivate metallic catalysts such as Ziegler catalysts and/or (3) do not have a critical surface tension low enough to effectively prevent polymer build-up.
Therefore, it is highly desirable to provide an improved coating and a process for using same which will significantly reduce the undesirable deposition of material on internal surfaces of reactors for a wide variety of chemical reactions.